How to Change Areas of Truss Elements

You can always increase the FOM of a truss by increasing the areas of the highest stressed elements and decreasing the areas of the lowest stressed elements or both. Ideally, all elements should have the design stress. Remember, you are building a model. The small increase in weight caused by the increase in area of a few elements is unimportant since the model will be scaled up to full size. If you find that after you change areas, the FOM stays the same or decreases, you made a mistake.

After running Ansys on a truss, Change the cross-sectional area of each element so its stress will be close to 16 ksi. Increase the area of those elements with stress greater than 16 ksi and decrease the area of those with stress less than 16 ksi by defining new R statements in your program. Since several elements will have the same stress because of symmetry, you only need a few R commands. Do not use duplicate R values.

Since we do not know the elastic modulus to better than 3%, your optimized stresses can be that far from the design value. Compare the class file trusswop.a to trussw.a to see what you need to do. For example, the lines:

      R,1,(3.0*7.227/16) ! area for elements 1-2
      R,2,(3.0*19.86/16) ! area for elements 3-4
change the areas of elements defined after the REAL,1 and REAL,2 commands. In the formula, 3.0 is the original area, and 7.227 is the original stress for REAL,1 elements. This line will increase the stress to 16 ksi. The second line reduces the stress of REAL,1 elements from 19.86 ksi to 16 ksi. (Note: Ansys interprets a * as a comment, so be sure to surround mathematical operations by parentheses.) Then use
      REAL,1
      E, ...        ! elements 1 and 2 in this example
      REAL,2
      E, ...        ! elements 3 and 4 in this example
to change the areas of the appropriate element

When you are done, check that the new stresses match the design stress.

MENG 421 -- Bridge -- Ansys index -- Class assignments

Last revised: November 19, 2003